In 1256, a great battle was fought between the O'Rourkes and the O'Reillys near Ballinamore. This led to the division of Breifne between the O'Rourkes and O'Reillys. The Bréifne region was split into East Bréifne and West Bréifne.

The Ó Ruairc kings (O'Rourke) maintained lordship over West Bréifne (mainly Co. Leitrim). The Kingdom of Bréifne region was part of the kingdom of Connacht up until the time of Queen Elizabeth I. In that time it was shired into the modern counties Cavan and Leitrim, Leitrim remaining a part of the province of Connacht while Cavan became part of Ulster.

The Ó Ruaircs were effectively lords of Breifne O'Rourke through the turbulent 16th century.

In ancient times the area that became to be known as Bréifne was said to be occupied by the Erdini, called in Irish 'Ernaigh', who possessed the entire country bordering Lough Erne.

At the time of the Christianization of Ireland (c. 5th–6th century) tribal groupings believed to be in or near Breifne included the Glasraighe, Masraige, Dartraige, Armhaighe, Gallraighe, the Fir Manach, and the Gailenga.

Around the 6th century a people known as the Conmaicne Rein are thought to have moved north from around the present Dunmore in County Galway and settled in Magh Rein (the area around Fenagh). From here they peopled what is now South Leitrim, which became known as Magh Rein, and its inhabitants as the Conmaicne Magh Rein. They consisted of different family groupings – Muintir Eolais, Muintir Cearbhallain (O Mulvey), and Cinel Luachain, among others.

About the 8th century, the area since known as Breifne was conquered and settled by the Uí Briúin who were a branch of the royal family of Connacht. The Uí Briúin established themselves first in modern county Leitrim and then into what is now County Cavan. It can be argued that there is no contemporary evidence to support these speculations.[citation needed]

By the 9th century the O'Ruaircs had established themselves as kings of Breifne.

In the 10th and 11th centuries the O'Ruairc kings of Breifne fought some battles for the title of king of Connacht, four different kings of Breifne gaining the title.

During the 12th century the reign of Tighearnán Ua Ruairc, the kingdom of Breifne was said to comprise most of the modern counties of Leitrim and Cavan, and parts of Longford, Meath, Fermanagh and Sligo.

In the 16th century Breifne O'Rourke eventually became Co. Leitrim and Breifne O'Reilly became Co. Cavan.

Duncarbry (Dun Chairbre) marks the border of Cairbre's territory on the Drowes, while the Barony of Carbury in North Sligo still reminds us also today.

Noted chiefs of Cenél Cairpre included O'Mulclohy (Ó Maolchloiche).

Cairpre Gabra – northern Co. Longford, barony of Granard. O'Ronan (Ó Ronáin) were chieftains in the barony of Granard until dispossessed by the O'Farrells in the 13th century.

According to the Annals of the Four Masters about 476 AD, the battle of Granard was fought by Eochaidh, son of Cairbre, son of Oilioll, son of Dunlaing, son of Enda Niadh against the Ui Dunlainge, Ui Briúin Cualann and Ui Fergusa of North Leinster.

Cenél Laegaire – Co. Fermanagh. The Fir Manach, the Cinéal Eanna and the Cenél Laegaire were early indigenous tribes in the Co. Fermanagh area. The Cenél Laegairi mic Neill were noted west of Loch Erne (Book of Lecan). The Cenél Laegairi mic Neill were also noted in central Ireland (Mide, Meath).